The Physics of Tires

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You can see that when you combine the vector of maximum acceleration with the vector of maximum cornering, you exceed the limit of the traction circle.  In other words, the car slides. In a front-wheel-drive car, this is understeer; it is power oversteer in the case of rear-wheel-drive.  That’s why you can’t go all-out on braking or accelerating during cornering; you have to have a mixture of both, like so: 

This diagram represents accelerating out of a corner.  As you straighten the wheel and corner less, you can gradually apply the accelerator more and more because you are using less cornering (lateral) grip.

See how those vectors move around?  In order to accelerate harder without sliding, you need to corner less.  In order to corner harder, you need to accelerate less.

A Real-World Example

My Mustang came with Pirelli P-Zero Nero all-season tires from the factory.  Don’t get me wrong; these are very good tires.  They are more than capable of handling whatever you could possibly face on the street.  Heck, it snowed 4 inches the week after I bought the car, and I was able to drive through the snow just fine without wrecking and/or dying.  But on the autocross grid, I was beginning to reach the limits of these tires already, even though I’m a novice.  During long, higher-speed corners, the tires would begin to lose their grip and understeer.

These tires are absolutely amazing for every day driving, and I would recommend them in a heartbeat for anyone not concerned with all-out performance.

The solution, of course, is to get stickier tires.  Stickier tires, in terms of what we’ve been talking about, simply make those traction circles bigger.  They allow you to do more accelerating, braking, and cornering before you reach the limit of the tires’ grip.  This is mainly accomplished through increasing the coefficient of friction.

I shopped around for a while for wheels and tires, because I wanted a good-looking, well-made, lightweight wheel.  And I was also broke and needed to save up money.

I decided on some rotary-forged wheels by TSW.  TSW named these wheels Nurburgring after the most awesome toll-road in the entire world.  I chose these tires because they had the exact look I wanted, and they are a very reasonable price.  My stock wheels were 18” x 7” so I went with 18” x 8” in the new wheels.  I could have fit 9-inch wide wheels, but I want to keep the weight (and the cost of tires) as low as possible.  The new wheels are about 18.6 lbs each, whereas the stock wheels were a little over 30.  That means I saved over 12 lbs of unsprung weight at each corner.

The fact that the wheels match the car’s color is an added bonus.

The tires I settled on are Nitto NT555s.  Nitto does make stickier street tires than this (the NT05s that come equipped on Steeda-tuned Mustangs), but the ‘555s have a treadwear rating of 300 as opposed to the ’05’s rating of 200.  These tires are also my daily-driving set, so I wanted a good treadlife.  Additionally, having a treadwear rating lower than 300 increases my time multiplier with the autocross club I run with.

These tires have a pretty aggressive tread on them without sacrificing tread life or wet traction.  Both of those are a plus.

Because I am a “polisher” in addition to a corner carver, I was compelled to replace the chrome “5.0” and “GT” badges with black ones.